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You DON'T apply positive power into a main ground with system dead, not powered up it has the potential to conduct reverse polarity literally anywhere. Anything with no protection against it can fry. To do that correctly BOTH cables cannot be hooked to battery. Take PCM, you just applied power to the ground and it runs down all grounds to enter PCM backwards and then exits out some of the constant on power for memory objects to get back to battery. It doesn't get any more reverse than that. You would need reverse polarity protection at a thousand points there, you don't have it.

The idea there is to decay all power to kill codes, you don't decay anything if the full power source is still connected. You can get the exact same effect by disconnecting one cable and waiting say 30 minutes.

FYI, I've watched 'that guy' remove filthy valve covers to simply turn them all over the place to drop dirt into the space under the valve covers that can lead to engine damage. While he is smart, he dismisses a lot of risk as nothing. The typical Ford mech who fixes your issue but suddenly you have a new one that came out of nowhere. Well, it didn't, there were reasons for it.
 
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